Fluid Delivery System and Method

ABSTRACT

A fluid delivery system includes a reservoir, a pump, an injector, and a pressure regulator. The reservoir encloses a fluid and includes a reservoir body forming a reservoir volume that encloses the fluid. The reservoir includes a draw conduit and a return conduit. The pump has an inlet connected to the draw conduit and provides fluid at the operating pressure and at a desired flow to a pressure line that is fluidly connected to an outlet of the pump. The injector selectively opens to allow an injected fluid flow to pass therethrough. The return orifice fluidly connects the pressure line with the return conduit, and the pressure regulator provides a regulated fluid flow to the fluid reservoir. During operation, the desired fluid flow is equal to a sum of the injected fluid flow, the return fluid flow and the regulated fluid flow.

TECHNICAL FIELD

This disclosure relates generally to fluid delivery systems used in association with vehicles and, more particularly, to a fluid delivery system for diesel exhaust fluid for use with engine emission control systems.

BACKGROUND

One known method for abating certain diesel engine exhaust constituents is by use of an exhaust after-treatment system that utilizes Selective Catalytic Reduction (SCR) of nitrogen oxides. In a typical SCR system, diesel exhaust fluid (DEF), which may include urea or a urea-based water solution, is mixed with exhaust gas before being provided to an appropriate catalyst. In some applications, the DEF is injected directly into an exhaust passage through a specialized injector device. In the case of urea, the injected DEF mixes with exhaust gas and breaks down to provide ammonia (NH₃) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NO_(x)) in the exhaust at a catalyst to provide nitrogen gas (N₂) and water (H₂O).

As can be appreciated, SCR systems require the presence of some form of DEF sufficiently close to the engine system such that the engine can be continuously supplied during operation. Various DEF delivery systems are known and used in engine applications. In known DEF injection systems, a reservoir is installed onto a vehicle for containing the DEF, which is drawn from the reservoir and delivered in metered amounts to the engine exhaust system.

In most engine applications, a precise delivery of DEF is required to achieve a desired and sufficient abatement of undesirable exhaust constituents as well as to avoid frequent fluid replenishments. For example, a fluid flow that is below a desired rate, depending on engine operating conditions, may not sufficiently abate engine emissions. Similarly, a fluid flow that is above a desired rate may deplete fluid supply in the vehicle prematurely, which can lead to more frequent vehicle service and/or insufficient emissions abatement due to lack of fluid after the fluid has been prematurely depleted and before it can be replenished.

SUMMARY

The disclosure describes, in one aspect, a fluid delivery system. The fluid delivery system includes a fluid reservoir adapted to enclose a fluid therewithin, the fluid reservoir comprising a reservoir body forming a reservoir volume that encloses the fluid therewithin and that includes a fluid draw conduit, which is configured to draw fluid from the reservoir volume, and a fluid return conduit, which is configured to return fluid to the reservoir volume. The fluid delivery system further includes a pump having an inlet fluidly connected to the fluid draw conduit such that the pump can draw fluid from the fluid reservoir, increase a pressure of the fluid to an operating pressure, and provide fluid at the operating pressure and at a desired fluid flow to a pressure line that is fluidly connected to an outlet of the pump. The fluid delivery system also includes a fluid injector fluidly in communication with the pressure line, the fluid injector configured to selectively open and allow pressurized fluid at a predetermined, injected fluid flow to pass therethrough when the fluid injector is open. A return orifice fluidly connecting the pressure line at a location downstream of the fluid injector with the fluid return conduit such that a return fluid flow is returned to the fluid reservoir can optionally be used, but is not required for all embodiments. A pressure regulator having a regulator inlet in fluid communication with the pressure line and a regulator outlet in fluid communication with the fluid return conduit is configured to provide a regulated fluid flow to the fluid reservoir when the operating pressure exceeds a pressure regulator opening pressure. During operation, the desired fluid flow is equal to a sum of the injected fluid flow, the return fluid flow and the regulated fluid flow.

In another aspect, the disclosure describes an exhaust after-treatment system for a machine. The system includes a diesel exhaust fluid (DEF) container adapted to enclose a DEF fluid therewithin. The DEF container comprises a reservoir body forming a reservoir volume that encloses the DEF and that includes a DEF draw conduit, which is configured to draw DEF from the reservoir volume, and a DEF return conduit, which is configured to return DEF to the reservoir volume. A DEF injector is configured to inject DEF from the container into an exhaust passage of an engine. A pump has an inlet fluidly connected to the DEF draw conduit such that the pump can draw DEF from the fluid reservoir and provide it at an operating pressure to the DEF injector through a pressure line. A return line has a return orifice and configured to return unused DEF from the DEF injector to the DEF container. A pressure regulator is configured to maintain a fluid pressure of the DEF provided to the DEF injector substantially constant by continuously shunting DEF from an outlet of the DEF pump to the DEF container. During operation, a DEF flow provided by the pump is equal to a DEF flow injected by the DEF injector, a second DEF flow returned to the DEF container, and a third DEF flow shunted to the DEF container by the pressure regulator.

In yet another aspect, the disclosure describes a method for operating a fluid system. The method includes drawing fluid from a reservoir with a pump, pressurizing the fluid with the pump to provide a desired fluid flow to a pressure line, circulating a return flow of fluid from the pressure line back to the reservoir through a return orifice continuously during operation, selectively injecting an injected flow of fluid from the pressure line through a fluid injector, shunting a regulated flow of fluid of fluid from the pressure line, and returning the regulated flow back to the reservoir continuously during operation, and adjusting, over a long term, the desired flow by comparing a pressure of fluid in the pressure line with a desired pressure. The desired flow is selected, based on environmental variables, and selectively set by appropriately commanding the pump. At all times during operation, the desired flow is equal to the sum of the return flow, the injected flow, and the regulated flow, not considering any leakage of fluid or other fluid loss from the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an engine having a SCR system in accordance with the disclosure.

FIG. 2 is a cross section of a fluid reservoir in accordance with the disclosure.

FIG. 3 is a schematic representation of a fluid delivery system in accordance with the disclosure.

FIG. 4 is a flowchart for a process in accordance with the disclosure.

DETAILED DESCRIPTION

This disclosure relates to emission control systems for engines and, more particularly, to DEF metering and delivery systems for use with SCR-based after-treatment systems for diesel engines used on stationary or mobile machines. The machines contemplated in the present disclosure can be used in a variety of applications and environments. For example, any machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, marine or any other industry known in the art is contemplated. For example, the type of machine contemplated herein may be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, material handler, locomotive, paver or the like. Apart from mobile machines, the machine contemplated may be a stationary or portable machine such as a generator set, an engine driving a gas compressor or pump, and the like. Moreover, the machine may include or be associated with work implements such as those utilized and employed for a variety of tasks, including, for example, loading, compacting, lifting, brushing, and include, for example, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and others.

FIG. 1 is a block diagram of an exhaust after-treatment system 101 associated with an engine 102 of a machine 100. The system 101 may be modularly packaged as shown in the illustrated embodiment for retrofit onto existing engines or, alternatively, for installation on new engines. In the illustrated embodiment, the system 101 includes a first module 104 that is fluidly connected to an exhaust conduit 106 of the engine 102. During engine operation, the first module 104 is arranged to internally receive engine exhaust gas from the conduit 106. The first module 104 may contain various exhaust gas treatment devices such as a diesel oxidation catalyst (DOC) 108 and a diesel particulate filter (DPF) 110, but other devices may be used. The first module 104 and the components found therein are optional and may be omitted for various engine applications in which the exhaust-treatment function provided by the first module 104 is not required. In the illustrated embodiment, exhaust gas provided to the first module 104 by the engine 102 may first pass through the DOC 108 and then through the DPF 110 before entering a transfer conduit 112.

The transfer conduit 112 fluidly interconnects the first module 104 with a second module 114 such that exhaust gas from the engine 102 may pass through the first and second modules 104 and 114 in series before being released at a stack 120 that is connected to the second module. In the illustrated embodiment, the second module 114 encloses a SCR catalyst 116 and an Ammonia Oxidation Catalyst (AMOX) 118. The SCR catalyst 116 and AMOX 118 operate to treat exhaust gas from the engine 102 in the presence of ammonia, which is provided after degradation of a urea-containing solution injected into the exhaust gas in the transfer conduit 112.

More specifically, a urea-containing water solution, which is commonly referred to as diesel exhaust fluid (DEF) 121, is injected into the transfer conduit 112 by a DEF injector 122. The DEF 121 is contained within a reservoir 128 and is provided to the DEF injector 122 by a pump 126. As the DEF 121 is injected into the transfer conduit 112, it mixes with exhaust gas passing therethrough and is carried to the second module 114. To promote mixing of DEF with exhaust, a mixer 124 may be disposed along the transfer conduit 112. The amount of DEF that may be injected into the transfer conduit 112 may be appropriately metered based on engine operating conditions. Accordingly, a desired amount of fluid at desired times may be provided to the transfer conduit 112 by the DEF delivery system.

As can be appreciated, the location of the DEF injector 122 on the transfer conduit 112 can expose the injector to relatively high temperatures due to heating from exhaust gas during operation. In the illustrated exemplary embodiment, a flow of engine coolant is provided through the injector, but such coolant flow is optional. Alternatively, DEF may be used as a coolant medium.

A cross section of one embodiment for the urea container or delivery reservoir 128 is shown in FIG. 2. In this embodiment, the reservoir is denoted by reference numeral 200. To further facilitate the expedient filling of the reservoir 200, in the installation shown in FIG. 2, an air gap or fill-vent opening 414 may be provided. In the illustrated embodiment, the fill-vent opening 414 is formed by a gap provided between inner and outer cylindrical walls 416 and 418 that are concentrically disposed within the fill opening of the reservoir 200. As shown, the inner and outer walls 416 and 418 extend at the same distance, D, within the reservoir 200 to define a maximum reservoir fill level. In this way, when the fluid 420 reaches the fill level at the height D below the top wall of the reservoir 200, the venting will cease and the operator will know that the reservoir has been filled to capacity. The desired fill level of the reservoir may be selected on numerous parameters such as reservoir capacity and the like, and may also provide a free space 422 at the top of the reservoir to account for fluid expansion due to heating and/or freezing without damaging the reservoir walls. A sock filter 428 surrounds the heater 426 and the pickup tube or fluid draw conduit 424.

Fluid 420 may be drawn from the reservoir 200 via a draw line 424. The draw line 424 may dray fluid from the bottom of the reservoir 200 and be surrounded by a heater 426 that can effectively melt frozen DEF fluid under cold operating conditions such that liquid DEF can be provided at a supply outlet 427. The liquid DEF at the supply outlet 427 may be delivered to a pump, for example, the pump 126 shown in FIG. 1.

A schematic of one embodiment for a fluid delivery system 300 is shown in FIG. 3. This embodiment, and especially the DEF filtering shown are exemplary and should not be understood as limiting. The delivery system 300 includes a reservoir 302, for example, the reservoir 200 as shown in FIG. 2, which contains DEF fluid for use by the system 300. A fluid draw conduit 304 is disposed within the reservoir 302 and arranged and configured to draw DEF fluid from therewithin. A staged filter arrangement includes an outer filter 306, such as the sock filter 428 shown in FIG. 2, and a secondary filter 308 disposed along the fluid draw conduit 304. Fluid drawn from the draw conduit 304 is provided to a suction line 310 that includes a primary filtration device 312. Filtered fluid from the suction line 310 is provided to a DEF pump 314. The DEF pump 314 may be enclosed in a housing 316 that includes a motor 318 connected to a pump 320. The pump 320 may be a variable or fixed displacement pump operating at a variable or fixed speed depending on system configuration, as will be discussed hereinafter. An internal filter 322 may further filter the fluid before the same enters the pump 320. A pressure sensor 324 disposed to measure fluid pressure at the outlet of the pump 320 is configured to provide a pressure signal indicative of a fluid pressure at the pump outlet to a controller 326 associated with the system 300.

Pressurized fluid at the outlet of the pump 320 is provided to a pressure line 328. The pressure line 328 as shown in the illustrated embodiment includes a pressure junction 330 that provides, in parallel fluid circuit arrangement, fluid at pump pressure to a pressure regulator 332 and to a DEF injector 334, for example, the DEF injector 122 (FIG. 1). During operation, a continuous flow of DEF fluid passes through the pressure line 328 and through a return orifice 336, which is disposed downstream of the DEF injector 334, before being provided back to the reservoir 302 via a return line 338. In other words, in one embodiment, the pressure regulator and the return orifice are in parallel fluid connection between the pressure junction and the reservoir. Of course, the return orifice may be placed elsewhere in the system, in series with the pressure regulator, and at other locations. When the return orifice is placed along a circuit branch that includes the DEF injector, a continuous flow of DEF can also act to cool the injector. Such continuous fluid circulation can also act to maintain the fluid well mixed, can control the temperature of the fluid under certain operating conditions such as cold operation, as well as ensure that an ample supply of pressurized fluid is available to the DEF injector 334 at all times during operation. In this way, in the embodiment illustrated, when a predetermined amount of fluid is desired for injection from the injector 334, the controller 326, based on the pressure signal from the sensor 324, may send a command signal such as a Pulse Width Modulated (PWM) signal to open the injector 334 for a predetermined period to allow a predetermined amount of fluid to be injected thereby.

When fluid is injected from the injector 334, fluid pressure in the pressure line may decrease, especially if an appreciable amount of fluid is injected. Such pressure drop within the pressure line 328 will be indicated to the controller 326 by the sensor 324. In response, the controller 326 will command the motor 318 to activate the pump 320 to supply fluid into the pressure line 328 until the desired pressure is once again established in the pressure line 328. The initiation of the pump, however, as well as the activation and deactivation of the injector 334, typically causes pressure pulsations, for example, standing waves or a hydraulic pressure spike of fluid pressure within the pressure line 328. Such pressure fluctuations can interfere, at least temporarily, with the pressure signal readings from the sensor 324. Moreover, such pressure spikes may interfere with the calculations in the controller 326 of the amount of fluid injected through the injector 334 because such fluid pressure may be above or below the predetermined system pressure that exists under stable conditions within the pressure line 328. These and other effects in the system, which can cause instability and large fluctuations in system pressure, especially under conditions when high fluid amounts are being delivered therethrough in relatively quick succession during machine operation, which can ultimately lead to a greater or lesser fluid being provided through the injector 334 than what is desired.

To address such and other related fluid pressure issues, at least in part, the pressure regulator 332 is configured to, at least in part, mitigate high pressure spikes in the pressure line 328. As shown, the pressure regulator includes a valve element 340 that is biased in a closed position via a spring 342 and that, when open, fluidly bypasses the injector 334 by fluidly and directly connecting the pressure line 328 with the return line 338. Although a mechanical pressure regulator is shown, an electronic pressure regulator valve may alternatively be used, or a mechanical arrangement having a different configuration than the one shown in FIG. 3. In the illustrated embodiment, the spring constant of the spring 342 is selected to yield an opening pressure for the valve element 340 that is about the same or just above the normal operating pressure in the pressure line 328. Thus, pressure spikes may cause the automatic opening of the pressure regulator 332 and dispose of high pressure fluid by returning it to the reservoir 302. However, the pressure regulator 332 alone is not sufficient to maintain a stable, reliable pressure within the pressure line 328, and may further lead to system instability when large and relatively frequent pressure fluctuations are present. Moreover, the pressure regulator, by its operation principles, cannot address low pressure conditions, especially when the transient response of the pump 314, for example, owing to the transient response of the pumping element 320 and/or motor 318, is slow to respond to pressure drops in the system, for example, when large amounts of DEF are being provided to the machine through the injector 334.

These and other issues may be avoided by appropriately controlling the motor 318 with the controller 326 to drive the pump 320 such that an excess amount of fluid is provided to the pressure line 328. In one embodiment, the pump 320 is driven by the motor 318 at a predetermined speed and/or displacement, in general, at predetermined fluid flow rate, which exceeds the return flow into the reservoir 302 through the return orifice 336 and also causes the pressure regulator 332 to be in an open position even when the injector 334 is in a fully open condition. Stated differently, the pump 320 is driven to provide an excess fluid supply to the pressure line 328 that exceeds the maximum fluid flow demand of the system 300 by a predetermined about, for example, 10 or 15% above the maximum expected flow through the DEF injector when the fluid pressure in the system is at its maximum allowable value and the injector is fully open, i.e., when the injector duty cycle is at 100%. The fluid supply from the pump, therefore, is equal to the sum of fluid injected through the injector, fluid returned to the reservoir through the return orifice, and fluid shunted from the pressure regulator at all times during operation. Of course, this equality of fluid flows does not account for other fluid losses from the system such as leaks, evaporation and the like, or fluid stored in system components such as in the various conduits or within the fluid injector, which fluid storage may occur transiently and/or occur at system startup or shutdown but is otherwise stabilized during system operation.

The excessive fluid supply described above during stable system operation will not cause a concomitant fluid pressure increase in the pressure line 328 because of the action of the pressure regulator 332. In short, when the opening pressure of the pressure regulator 332 is selected to be about equal and, preferably, just below the desired fluid pressure under steady conditions within the pressure line 328, the excess fluid provided to the pressure line 328 will be shunted back to the reservoir 302 through the pressure regulator continuously during operation. In conditions when the injector 334 is open, the excess fluid flow provided by the pump 320 will account for the flow through the injector 334, the flow through the return orifice 336, and will also still cause the pressure regulator 332 to open, at least partially, to shunt fluid back to the reservoir 302. In this way, a stable pressure can be maintained at all times within the pressure line 328, and dampening that will reduce or eliminate pressure fluctuations within the pressure line 328 can be provided by a combination of the return orifice 336 and the flow through the pressure regulator 332.

To improve system control accuracy and avoid unnecessary wear on the pumping and other fluid elements of the system, the control scheme for the pump 314 operating within the controller 326 in the system 300 may account for various environmental and aging effects in the system. In one embodiment, the control algorithm, which provides a command to the motor 318 as an output, can include a closed-loop controller that is used to set the fluid flow rate of fluid provided through the pump 320 at a point that is just above the corresponding setting on the pressure regulator 332. In one contemplated embodiment, the closed-loop controller uses a feed-forward control term to set the initial pump speed to a predetermined pump speed that yields the desired fluid flow. The predetermined pump speed can be selected or set based on pump performance mapping and environmental conditions such as ambient temp, fluid temp, altitude and pressure setting of the pressure regulator. If such predetermined pump speed setting is considered as a base or normal operating condition, the control algorithm can also monitor system pressure and use a relatively long term feedback, for example, via an integral control term having a relatively large time constant, that is based on system pressure to slowly adapt and adjust the fluid flow rate through the pump and maintain predetermined and/or desired flow margin above a maximum flow consumption of the system. In this way, the pressure can automatically control overall system pressure.

A block diagram for a control 500 that controls the operation of the motor 318 and/or a displacement of the pump 320, as applicable to the system 300 as shown in FIG. 3, is illustrated in FIG. 4. As can be appreciated, the control 500 may adjust motor speed when the motor is associated with a fixed-displacement pump to control fluid flow, or may alternatively control a pump displacement in a variable displacement pump when the same is associated with a fixed-speed motor to control fluid flow. The control 500 may operate within the controller 326. The controller 326 may be a single controller or may include more than one controller disposed to control various functions and/or features of a machine. For example, a master controller, used to control the overall operation and function of the machine, may be cooperatively implemented with a motor or engine controller, used to control other machine systems, for example the engine 102. In this embodiment, the term “controller” is meant to include one, two, or more controllers that may be associated with the machine 100 (FIG. 1) and that may cooperate in controlling various functions and operations of the machine 100. The functionality of the controller 326, while shown conceptually in FIG. 4 to include various discrete functions for illustrative purposes only, may be implemented in hardware and/or software without regard to the discrete functionality shown. Accordingly, various interfaces of the controller are described relative to components of the system 300 (FIG. 3) shown in the block diagram of FIG. 4. Such interfaces are not intended to limit the type and number of components that are connected, nor the number of controllers that are described.

During operation, the controller 500 provides a motor/pump command signal 502, which as previously described may control the speed of a motor operating a pump and/or a displacement of a pump. In any case, the motor/pump command signal 502 is a signal that causes a change in a fluid flow provided to a pressure line of a fluid system such as the pressure line 328 in the system 300 (FIG. 3). Various signals are provided as inputs to the control 500, on the basis of which the command signal 502 is determined. In the illustrated embodiment, inputs to the control 500 include ambient temperature 504, fluid temperature 506 and altitude 508. These inputs, which are collectively considered environmental inputs, may include more or fewer parameters. An addition input to the control 500 is system pressure 510. The system pressure 510 is a signal indicative of the pressure of fluid at the outlet of the pump. One example of system pressure may be the pressure signal provided by sensor 324 (FIG. 3) to the controller 326 that is indicative of the real-time pressure of fluid within the pressure line 328.

The various environmental inputs, i.e., the ambient temperature 504, fluid temperature 506 and altitude 508 in the embodiment shown in FIG. 4, along with a constant 512 are provided to a desired system pressure determinator function 514. The constant 512 represents the default or designed-for opening pressure of the pressure regulator. The desired system pressure determinator function 514 in the illustrated embodiment comprises various lookup tables and compensation functions that provide, based on the then-present operating conditions of the system, an indication as to the desired pressure setting for the system. As can be appreciated, static pressure conditions such as altitude, and fluid temperature, may affect the reading of the sensor or other means used to monitor and provide an indication of the system pressure 510. To offset or compensate for such effects, as well as mechanical pump and motor effects due to temperature, the system pressure determinator function is pre-populated or pre-programmed to provide an indication of a desired system pressure 516 that is sufficient in the system to reflect an ample supply of fluid for the fluid injector and for the pressure regulator to bypass and achieve stable system performance as described above.

The desired system pressure 516 is provided to a summing junction 518, where it is compared to the system pressure 510. A pressure difference or error 520, which is indicative of a difference between the desired system pressure 516 and the actual, measured or estimated system pressure 510 that is present in the system, is provided to an integral function 522. The integral function, may be of the form shown in Equation 1 below:

I(t) = K_(i)∫₀^(t)e(τ)∂t

where I(t) is the integral term over time (t), K_(i) is a constant, and e(τ) is a function that is integrated over a period (τ). As is known, integral terms can address residual steady-state error that can occur in systems. In this case, such errors may result from various sources such as sensor error, sensor creep, system aging, filter clogging, and other effects. In the illustrated embodiment, the pressure difference or error 520 may also be provided to a sentry function 524 that can provide a system fault signal 526 indicating that system service is required or that notifies the operator of a fault when the error 520 exceeds a maximum allowable error for a predetermined period.

The integral function 522 provides a correction signal 528 which passes through a delimiter 530. A delimited correction signal 532 and the desired system pressure 516 are provided to a summing junction 534 and are compounded to provide a corrected, desired system pressure in the form of the command signal 502. As can be appreciated, the desired system pressure 516, which can also be expressed as a desired system flow rate setpoint, is independent of fluid use by the system and only depends on fixed system parameters such as pump and motor operation and, optionally, on environmental parameters within which the system is operating. The flow and/or pressure setpoint provided by the determinator function 514 is independent of fluid use, which leads to an inherently stable control scheme. As previously discussed, flow changes within the system are addressed by the pressure regulator such that there is always a flow excess provided to the system. The steady-state error compensation provided by the integral function 522 addresses effects that may appear in the system over time and also helps diagnose system faults.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to emission control systems for engines and, more particularly, to emission control systems using SCR processes requiring the injection of urea-based water solutions into engine exhaust streams. In the disclosed embodiments, a feed forward controller having a long-term feedback is used to create a control arrangement in which pressure fluctuations in the high-pressure DEF fluid delivery system are avoided. In one embodiment, the system sets a predetermined DEF flow which exceeds the maximum use of DEF by the injector such that an excess flow causes a pressure regulator to open, at all times, thus controlling the pressure continuously within the system.

It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 

We claim:
 1. A fluid delivery system, comprising: a fluid reservoir adapted to enclose a fluid therewithin, the fluid reservoir comprising a reservoir body forming a reservoir volume that encloses the fluid therewithin and that includes a fluid draw conduit, which is configured to draw fluid from the reservoir volume, and a fluid return conduit, which is configured to return fluid to the reservoir volume; a pump having an inlet fluidly connected to the fluid draw conduit such that the pump can draw fluid from the fluid reservoir, increase a pressure of the fluid to an operating pressure, and provide fluid at the operating pressure and at a desired fluid flow to a pressure line that is fluidly connected to an outlet of the pump; a fluid injector fluidly in communication with the pressure line, the fluid injector configured to selectively open and allow pressurized fluid at a predetermined, injected fluid flow to pass therethrough when the fluid injector is open; and a pressure regulator having a regulator inlet in fluid communication with the pressure line and a regulator outlet in fluid communication with the fluid return conduit, the pressure regulator configured to provide a regulated fluid flow to the fluid reservoir when the operating pressure exceeds a pressure regulator opening pressure; wherein, during operation, the desired fluid flow is equal to a sum of the injected fluid flow and the regulated fluid flow.
 2. The fluid delivery system of claim 1, further comprising: a pressure sensor disposed in fluid communication with the pressure line and configured to measure a fluid pressure within the pressure line and provide a pressure signal; and an electronic controller disposed to receive the pressure signal.
 3. The fluid delivery system of claim 2, wherein the pump is a fixed displacement pump that is operated by a variable speed motor, the variable speed motor being responsive to a command signal provided by the electronic controller based, at least in part, on the pressure signal.
 4. The fluid delivery system of claim 3, wherein the command signal provided by the electronic controller uses environmental variables as primary control parameters to determine the command signal that will provide the desired fluid flow to the pressure line.
 5. The fluid delivery system of claim 4, wherein the environmental variables include ambient temperature, fluid temperature, and altitude.
 6. The fluid delivery system of claim 3, wherein the electronic controller includes an integrator function that provides a compensation factor based on a difference between a desired pressure and the operating pressure, said compensation factor having a long term.
 7. The fluid delivery system of claim 2, wherein the motor has a fixed speed and the pump has a variable displacement such that the pump is a variable displacement pump, the variable displacement pump being responsive to a command signal provided by the electronic controller based, at least in part, on the pressure signal.
 8. The fluid delivery system of claim 7, wherein the command signal provided by the electronic controller uses environmental variables as primary control parameters to determine the command signal that will provide the desired fluid flow to the pressure line.
 9. The fluid delivery system of claim 1, further comprising a return orifice fluidly connecting the pressure line at a location downstream of the fluid injector with the fluid return conduit such that a return fluid flow is returned to the fluid reservoir, wherein, during operation, the desired fluid flow is equal to a sum of the injected fluid flow, the return fluid flow and the regulated fluid flow.
 10. The fluid delivery system of claim 7, wherein the electronic controller includes an integrator function that provides a compensation factor based on a difference between a desired pressure and the operating pressure, said compensation factor having a long term.
 11. The fluid delivery system of claim 1, wherein the pressure line is part of a fluid flow circuit that includes a pressure junction disposed between the pressure regulator and the fluid injector such that the pressure regulator is disposed in parallel circuit connection with the return orifice between the pressure junction and the fluid reservoir.
 12. The fluid delivery system of claim 1, wherein the pressure regulator includes a valve element that is biased in a closed position by a spring, and wherein the spring is selected to have a spring constant that enables the valve element to open and fluid connect the pressure line with the reservoir when a pressure of fluid upstream of the valve element exceeds a force applied on the valve element by the spring.
 13. An exhaust after-treatment system for a machine, comprising: a diesel exhaust fluid (DEF) container adapted to enclose a DEF fluid therewithin, the DEF container comprising a reservoir body forming a reservoir volume that encloses the DEF and that includes a DEF draw conduit, which is configured to draw DEF from the reservoir volume, and a DEF return conduit, which is configured to return DEF to the reservoir volume; a DEF injector configured to inject DEF from the container into an exhaust passage of an engine; a pump having an inlet fluidly connected to the DEF draw conduit such that the pump can draw DEF from the container and provide it at an operating pressure to the DEF injector through a pressure line; a return line configured to return unused DEF from the DEF injector to the DEF container; and a pressure regulator configured to maintain a fluid pressure of the DEF provided to the DEF injector substantially constant by continuously shunting DEF from an outlet of the DEF pump to the DEF container; wherein, during operation, a DEF flow provided by the pump is equal to a DEF flow injected by the DEF injector, a second DEF flow returned to the DEF container, and a third DEF flow shunted to the DEF container by the pressure regulator.
 14. The exhaust after-treatment system for a machine of claim 1, further comprising: a pressure sensor disposed in fluid communication with the pressure line and configured to measure a DEF pressure within the pressure line and provide a pressure signal; and an electronic controller disposed to receive the pressure signal.
 15. The exhaust after-treatment system for a machine of claim 14, wherein the pump has a fixed displacement and is operated by a variable speed motor, the variable speed motor being responsive to a command signal provided by the electronic controller based, at least in part, on the pressure signal, wherein the command signal provided by the electronic controller uses environmental variables as primary control parameters to determine the command signal that will provide the desired fluid flow to the pressure line, and wherein the environmental variables include ambient temperature, fluid temperature, and altitude.
 16. The exhaust after-treatment system for a machine of claim 15, wherein the electronic controller includes an integrator function that provides a compensation factor based on a difference between a desired pressure and the operating pressure, said compensation factor having a long term.
 17. The exhaust after-treatment system for a machine of claim 14, wherein the motor has a fixed speed and the pump has a variable displacement such that the pump is a variable displacement pump, the variable displacement pump being responsive to a command signal provided by the electronic controller based, at least in part, on the pressure signal, wherein the command signal provided by the electronic controller uses environmental variables as primary control parameters to determine the command signal that will provide the desired fluid flow to the pressure line, and wherein the environmental variables include ambient temperature, fluid temperature, and altitude.
 18. The exhaust after-treatment system for a machine of claim 17, wherein the electronic controller includes an integrator function that provides a compensation factor based on a difference between a desired pressure and the operating pressure, said compensation factor having a long term.
 19. The exhaust after-treatment system for a machine of claim 14, wherein the pressure line is part of a fluid flow circuit that includes a pressure junction disposed between the pressure regulator and the return orifice such that the pressure regulator is disposed in parallel circuit connection with the return orifice between the pressure junction and the fluid reservoir.
 20. A method for operating a fluid system, comprising: drawing fluid from a reservoir with a pump; pressurizing the fluid with the pump to provide a desired flow to a pressure line; circulating a return flow of fluid from the pressure line back to the reservoir through a return orifice continuously during operation; selectively injecting an injected flow of fluid from the pressure line through a fluid injector; shunting a regulated flow of fluid of fluid from the pressure line, and returning the regulated flow back to the reservoir continuously during operation; and adjusting, over a long term, the desired flow by comparing a pressure of fluid in the pressure line with a desired pressure; selecting, based on environmental variables, the desired flow and selectively setting the same by appropriately commanding the pump; wherein, at all times during operation, the desired flow is equal to the sum of the return flow, the injected flow, and the regulated flow. 